1. Technical Field
The present invention relates to a radio transmitting apparatus and radio transmission method for carrying out RACH (Random Access Channel) transmission.
2. Description of the Related Art
In 3GPP RAN LTE (Long Term Evolution), studies are being conducted for non-synchronous random access channel (hereinafter referred to as “Async RACH”). Async RACH refers to random access transmitted in a state where uplink synchronization is not established and used in acquiring transmission timings of a mobile station (i.e., uplink synchronization establishment) and reporting a signature (i.e., mobile station identification information) to a base station.
In LTE, studies are being conducted for the structure and transmission method of a preamble transmitted initially in Async RACH, and, by including 4 to 8 bits of control information in a preamble signal in addition to a signature (i.e., mobile station identification information: ID), resource (i.e., time and frequency) use efficiency improves (e.g., see non-patent document 1).
At least a signature is transmitted in the preamble signal. A code pattern of good correlation characteristic with the signature is uniquely associated with the signature in advance, and the mobile station transmits the code pattern associated with the signature selected on a random basis as the preamble signal. By finding the correlations between all code patterns that can be transmitted and a received signal, the receiving side (base station) can detect different signatures at the same time. Further, studies are being underway to improve resource use efficiency by transmitting control information uniquely associated with a signature (e.g., see non-patent documents 2 and 3).
The control information to be transmitted in the preamble includes DL CQI (Downlink Channel Quality Indicator), RACH cause (the purpose of use and the reason for transmission of RACH and RACH access type) and so on (e.g., see non-patent documents 2 and 3).
FIG. 1 shows the steps from RACH transmission to start of data communication described in non-patent document 3. Effects for transmitting control information will be explained as follows.
Immediately after RACH preamble, transmission timing information, DL CH for transmitting resource assignment information and UL CH for transmitting a scheduling request are transmitted. Transmitting DL CQI makes it possible to select an MCS (Modulation and Coding set) according to received quality conditions of UL/DL CH transmitted immediately after RACH preamble. That is, for mobile stations in good reception environment, by selecting an MCS of low redundancy (e.g., 16 QAM and high coding rate), it is possible to use resources (time and frequency band) effectively between a plurality of mobile stations.
Moreover, in LTE, the access steps until start of data communication vary according to the purpose of use and the reason for transmission of the RACH, so that, by transmitting a RACH access type (i.e., RACH cause) in the preamble, it is possible to send only information that matches the purpose of use of the RACH, in UL/DL CH immediately after RACH preamble. Consequently, useless information transmission can be omitted and resource use efficiency improves.
In non-patent document 3, transmitting five-bit control information (i.e., two-bit DL CQI+three-bit RACH cause) by a preamble is proposed. To be more specific, as shown in FIG. 2, by associating the control information with a signature, five-bit control information can be transmitted by the signature.    Non-patent Document 1: 3GPP TR25.814 V1.5.0, 9.1.2.1.1.2    Non-patent Document 2: R1-061184, NTT DoCoMo    Non-patent Document 3: R1-061393, Texas Instruments